public void EditXmlFile()
        {
            XmlDocument doc = new XmlDocument();
            doc.Load(@"E:\Test.xml");
            XmlNode xmlNde = doc.DocumentElement.SelectSingleNode(@"FirstChild/Address");
            xmlNde.InnerText = "Ernakulam";
            doc.Save(@"E:\Test.xml");
            doc.RemoveAll();
        }

<DocElem>
<FirstChild>
<Name>Sabu</Name>
<Address>Cochin</Address>
<Age>28</Age>
<Nationality>Indian</Nationality>
</FirstChild>
</DocElem>